Electronic devices containing active organic materials are attracting increasing attention for use in devices such as organic light emitting diodes (OLEDs), organic photoresponsive devices (in particular organic photovoltaic devices and organic photosensors), organic transistors and memory array devices. Devices containing active organic materials offer benefits such as low weight, low power consumption and flexibility. Moreover, use of soluble organic materials allows use of solution processing in device manufacture, for example inkjet printing or spin-coating.
An organic light-emitting device has a substrate carrying an anode, a cathode and one or more organic light-emitting layers between the anode and cathode. An exemplary anode material is indium tin oxide (ITO). Cathodes include a single layer of metal such as aluminium, a bilayer of calcium and aluminium as disclosed in WO 98/10621; and a bilayer of a layer of an alkali or alkali earth compound and a layer of aluminium as disclosed in L. S. Hung, C. W. Tang, and M. G. Mason, Appl. Phys. Lett. 70, 152 (1997); T. Hasegawa, S. Miura, T. Moriyama, T. Kimura, I. Takaya, Y. Osato, and H. Mizutani, SID Int. Symp. Digest. Tech. Papers 35, 154 (2004); R. Suhonen, R. Krause, F. Kozlowski, W. Sarfert, R. Patzold, and A. Winnacker, Org. Electron. 10, 280 (2009).
Holes are injected into the device through the anode and electrons are injected through the cathode during operation of the device. Holes in the highest occupied molecular orbital (HOMO) and electrons in the lowest unoccupied molecular orbital (LUMO) of a light-emitting material combine to form an exciton that releases its energy as light. Charge-transporting layers may be provided.
WO 2012/133229 discloses a polymer of formula (B):

A light emitting layer may comprise a semiconducting host material and a light-emitting dopant wherein energy is transferred from the host material to the light-emitting dopant. For example, J. Appl. Phys. 65, 3610, 1989 discloses a host material doped with a fluorescent light-emitting dopant—that is, a light-emitting material in which light is emitted via decay of a singlet exciton.
Phosphorescent dopants are also known—that is, a light-emitting dopant in which light is emitted via decay of a triplet exciton.
Dopants with a high energy excited state require a host material with a large HOMO-LUMO bandgap to prevent quenching of emission. However, the present inventors have found that this large HOMO-LUMO bandgap can affect the efficiency of electron transport into the light-emitting layer or layers.
It is an object of the invention to improve efficiency of organic light-emitting devices, in particular devices containing a high energy excited state emitter.
It is a further object of the invention to improve conductivity of organic light-emitting devices, in particular devices containing a high energy excited state emitter.